A brief sketch of these dependent changes
of colour may, however, be advantageously given here.
_Variable Protective Colouring._
There are two distinct kinds of change of colour in animals due to the
colouring of the environment. In one case the change is caused by reflex
action set up by the animal _seeing_ the colour to be imitated, and the
change produced can be altered or repeated as the animal changes its
position. In the other case the change occurs but once, and is probably
not due to any conscious or sense action, but to some direct influence
on the surface tissues while the creature is undergoing a moult or
change to the pupa form.
The most striking example of the first class is that of the chameleon,
which changes to white, brown, yellowish, or green, according to the
colour of the object on which it rests. This change is brought about by
means of two layers of pigment cells, deeply seated in the skin, and of
bluish and yellowish colours. By suitable muscles these cells can be
forced upwards so as to modify the colour of the skin, which, when they
are not brought into action, is a dirty white.
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